Each X will increase the oxidation number of metal by +1. Each L and X will supply 2 electrons to the electron count.

Now looking at compounds having a charge of +1 to obey 18 e rule. Elec count: 4 (M) +2 (NO) +12 (L6) = 18 NO+ is isoelectronic to CO X increases O N by 1 Elec Count: 4 (M) + 4 (L2) + 10 (L5)

Actors and spectators Actor ligands are those that dissociate or undergo a chemical transformation (where the chemistry takes place!) Spectator ligands remain unchanged during chemical transformations They provide solubility, stability, electronic and steric influence (where ligand design is !)

Organometallic Chemistry 1.2 Fundamental Reactions

Fundamental reaction of organo-transition metal complexes D(FOS) D(CN) D(NVE) Association-Dissociation of Lewis acids ±1 Association-Dissociation of Lewis bases ±2 Oxidative addition-Reductive elimination Insertion-deinsertion FOS: Formal Oxidation State; CN: Coordination Number NVE: Number of valence electrons

Association-Dissociation of Lewis acids D(FOS) = 0; D(CN) = ± 1; D(NVE) = 0 Lewis acids are electron acceptors, e.g. BF3, AlX3, ZnX2 This shows that a metal complex may act as a Lewis base The resulting bonds are weak and these complexes are called adducts

Association-Dissociation of Lewis bases D(FOS) = 0; D(CN) = ± 1; D(NVE) = ±2 A Lewis base is a neutral, 2e ligand “L” (CO, PR3, H2O, NH3, C2H4,…) in this case the metal is the Lewis acid Crucial step in many ligand exchange reactions For 18-e complexes, only dissociation is possible For <18-e complexes both dissociation and association are possible but the more unsaturated a complex is, the less it will tend to dissociate a ligand

D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2 Oxidative addition-reductive elimination D(FOS) = ±2; D(CN) = ± 2; D(NVE) = ±2 Vaska’s compound Very important in activation of hydrogen

Oxidative addition-reductive elimination H becomes H- Concerted reaction Vaska’s compound via Ir: Group 9 cis addition CH3+ has become CH3- SN2 displacement trans addition Also radical mechanisms possible

Oxidative addition-reductive elimination Not always reversible

Insertion-deinsertion D(FOS) = 0; D(CN) = 0; D(NVE) = 0 Mn: Group 7 Very important in catalytic C-C bond forming reactions (polymerization, hydroformylation) Also known as migratory insertion for mechanistic reasons

Migratory Insertion Also promoted by including bulky ligands in initial complex

Insertion-deinsertion The special case of 1,2-addition/-H elimination A key step in catalytic isomerization & hydrogenation of alkenes or in decomposition of metal-alkyls Also an initiation step in polymerization

Attack on coordinated ligands Very important in catalytic applications and organic synthesis

Some examples of attack on coordinated ligands Nucleophilic addition Electrophilic addition Nucleophilic abstraction Electrophilic abstraction

Organometallic Chemistry Brooklyn College Chem 76/76.1/710G Advanced Inorganic Chemistry (Spring 2009) Unit 6 Organometallic Chemistry Part 2. Some physical and chemical properties of important classes of coordination and organometallic compounds Suggested reading: Miessler/Tarr Chapters 13 and 14

Metal Carbonyl Complexes CO as a ligand s donor, π-acceptor strong trans effect small steric effect CO is an inert molecule that becomes activated by complexation to metals

“C-like MO’s” Frontier orbitals Larger homo lobe on C

Mo(CO)6 anti bonding “metal character” non bonding “18 electrons” 6CO ligands x 2s e each 12 s bonding e “ligand character”

Metal carbonyls may be mononuclear or polynuclear

Synthesis of metal carbonyls

Characterization of metal carbonyls IR spectroscopy (C-O bond stretching modes)

Effect of other ligands Effect of charge u(free CO) 2143 cm-1 Lower frequency, weaker CO bond Effect of other ligands PF3 weakest donor (strongest acceptor) PMe3 strongest donor (weaker acceptor)

The number of active bands as determined by group theory

13C NMR spectroscopy 13C is a S = 1/2 nucleus of natural abundance 1.108% 1.6% as sensitive as 1H only For metal carbonyl complexes d 170-290 ppm (diagnostic signals) Very long T1 (use relaxation agents like Cr(acac)3 and/or enriched samples)

Typical reactions of metal carbonyls Ligand substitution: Always dissociative for 18-e complexes, may be associative for <18-e complexes Migratory insertion:

Metal complexes of phosphines PR3 as a ligand Generally strong s donors, may be π-acceptor strong trans effect Electronic and steric properties may be controlled Huge number of phosphines available

Metal complexes of phosphines Basicity: PCy3 > PEt3 > PMe3 > PPh3 > P(OMe)3 > P(OPh)3 > PCl3 > PF3 Can be measured by IR using trans-M(CO)(PR3) complexes Steric properties: Rigid structures create chiral complexes apex angle of a cone that encompasses the van der Waals radii of the outermost atoms of the ligand

Tolman’s electronic and steric parameters of phosphines

Typical reactions of metal-phosphine complexes Ligand substitution: presence of bulky ligands (large cone angles) can lead to more rapid ligand dissociation Very important in catalysis Mechanism depends on electron count

Metal hydride and metal-dihydrogen complexes Terminal hydride (X ligand) Bridging hydride (m-H ligand, 2e-3c) Coordinated dihydrogen (h2-H2 ligand) Hydride ligand is a strong s donor and the smallest ligand available H2 as ligand involves -donation and π-back donation

Synthesis of metal hydride complexes

Characterization of metal hydride complexes 1H NMR spectroscopy High field chemical shifts (d 0 to -25 ppm usual, up to -70 ppm possible) Coupling to metal nuclei (101Rh, 183W, 195Pt) J(M-H) = 35-1370 Hz Coupling between inequivalent hydrides J(H-H) = 1-10 Hz Coupling to 31P of phosphines J(H-P) = 10-40 Hz cis; 90-150 Hz trans IR spectroscopy n(M-H) = 1500-2000 cm-1 (terminal); 800-1600 cm-1 bridging n(M-H)/n(M-D) = √2 Weak bands, not very reliable

Some typical reactions of metal hydride complexes Transfer of H- Transfer of H+ A strong acid !! Insertion A key step in catalytic hydrogenation and related reactions

Bridging metal hydrides Anti-bonding Non-bonding 2-e ligand 4-e ligand bonding

Metal dihydrogen complexes Characterized by NMR (T1 measurements) Very polarized d+, d- back-donation to s* orbitals of H2 the result is a weakening and lengthening of the H-H bond in comparison with free H2 If back-donation is strong, then the H-H bond is broken (oxidative addition)

Metal-olefin complexes 2 extreme structures sp3 sp2 Zeise’s salt metallacyclopropane π-bonded only Net effect weakens and lengthens the C-C bond in the C2H4 ligand (IR, X-ray)

Effects of coordination on the C=C bond Compound C-C (Å) M-C (Å) C2H4 1.337(2) C2(CN)4 1.34(2) C2F4 1.31(2) K[PtCl3(C2H4)] 1.354(2) 2.139(10) Pt(PPh3)2(C2H4) 1.43(1) 2.11(1) Pt(PPh3)2(C2(CN)4) 1.49(5) 2.11(3) Pt(PPh3)2(C2Cl4) 1.62(3) 2.04(3) Fe(CO)4(C2H4) 1.46(6) CpRh(PMe3)(C2H4) 1.408(16) 2.093(10) C=C bond is weakened (activated) by coordination

Characterization of metal-olefin complexes IR n(C=C) ~ 1500 cm-1 (w) NMR 1H and 13C, d < free ligand X-rays C=C and M-C bond lengths indicate strength of bond

Synthesis of metal-olefin complexes [PtCl4]2- + C2H4  [PtCl3(C2H4)]- + Cl- RhCl3.3H2O + C2H4 + EtOH  [(C2H4)2Rh(m-Cl)2]2

Reactions of metal-olefin complexes

Metal alkyl, carbene and carbyne complexes

Metal-alkyl complexes Main group metal-alkyls known since old times (Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903)) Transition-metal alkyls mainly from the 1960’s onward W(CH3)6 Ti(CH3)6 PtH(CCH)L2 Cp(CO)2Fe(CH2CH3)6 [Cr(H2O)5(CH2CH3)6]2+ Why were they so elusive? Kinetically unstable (although thermodynamically stable)

Reactions of transition-metal alkyls Blocking kinetically favorable pathways allows isolation of stable alkyls

Metal-carbene complexes L ligand Late metals Low oxidation states Electrophilic X2 ligand Early metals High oxidation states Nucleophilic

Fischer-carbenes

Schrock-carbenes Synthesis Typical reactions + olefin metathesis (we will speak more about that)

Grubbs carbenes Excellent catalysts for olefin metathesis

(“sandwich compounds”) Metal cyclopentadienyl complexes Metallocenes (“sandwich compounds”) Bent metallocenes “2- or 3-legged piano stools”

Homogeneous catalysis: an important application of organometallic compounds Catalysis in a homogeneous liquid phase Very important fundamentally Many synthetic and industrial applications

Comparison of heterogeneous and homogeneous catalysts Usually distinct solid phase Readily separated Readily regenerated and recycled Rates not usually as fast as homogeneous May be difussion limited Quite selective to poisons Lower selectivity Long service life Often high-energy process Poor mechanistic understnding Same phase as reaction medium Often difficult to separate Expensive/difficult to recycle Often very high rates Not diffusion controlled Usually robust to poisons High selectivity Short service life Often takes place under mild conditions Often mechanism well understood Difficulties in separation and catalyst regeneration have prevented a wider use of homogeneous catalysts in industry

Fundamental reaction of organo-transition metal complexes D(FOS) D(CN) D(NVE) Association-Dissociation of Lewis acids ±1 Association-Dissociation of Lewis bases ±2 Oxidative addition-Reductive elimination Insertion-deinsertion

Combining elementary reactions

b-H elimination resulting in C=C bond migration Completing catalytic cycles Olefin isomerization b-H elimination no net reaction b-H elimination resulting in C=C bond migration

Completing catalytic cycles Olefin isomerization

(reductive elimination) Completing catalytic cycles Olefin hydrogenation (reductive elimination)

Completing catalytic cycles Olefin hydrogenation

Wilkinson’s hydrogenation catalyst RhCl(PPh3)3 Very active at 25ºC and 1 atm H2 Very selective for C=C bonds in presence of other unsaturations Widely used in organic synthesis Prof. G. Wilkinson won the Nobel Prize in 1973

The mechanism of olefin hydrogenation by Wilkinson’s catalyst

Other hydrogenation catalysts [Rh(H)2(PR3)2(solv)2]+ With a large variety of phosphines including chiral ones for enantioselective hydrogenation RuII/(chiral diphosphine)/diamine Extremely efficient catalysts for the enantioselective hydrogenation of C=C and C=O bonds Profs. Noyori, Sharpless and Knowles won the Nobel Prize in 2001

Olefin hydroformylation Cat: HCo(CO)4; HCo(CO)3(PnBu3) HRh(CO)(PPh3)3; HRh(CO)(TPPTS)3 6 million Ton /year of products worldwide Aldehydes are important intermediates towards plastifiers, detergents

What else could happen if CO is present? Olefin hydrogenation (reductive elimination) What else could happen if CO is present? CO insertion reductive elimination

Olefin hydroformylation

Catalysts for polyolefin synthesis Polyolefins are the most important products of organometallic catalysis (> 60 million Tons per year) Polyethylene (low, medium, high, ultrahigh density) used in packaging, containers, toys, house ware items, wire insulators, bags, pipes. Polypropylene (food and beverage containers, medical tubing, bumpers, foot ware, thermal insulation, mats)

Catalytic synthesis of polyolefin

Catalytic synthesis of polyolefin High density polyethylene (HDPE) is linear, d  0.96 “Ziegler catalysts”: TiCl3,4 + AlR3 Vacant site Coordinated alkyl Electrophilic metal center Insoluble (heterogeneous) catalyst

Catalytic synthesis of polyolefin Isotactic polypropylene is crystalline “Natta catalysts”: TiCl3 + AlR3 Vacant site Coordinated alkyl Electrophilic metal center Insoluble (heterogeneous) catalyst, crystal structure determines tacticity

Catalytic synthesis of polyolefin “Kaminsky catalysts” Vacant site Coordinated alkyl Electrophilic metal center Soluble (homogeneous) catalyst, structural rigidity determines tacticity

Polymerization mechanism

The catalytic synthesis of acetaldehyde (Wacker process, oxidation of ethylene) Pd(0) + 2CuCl2 PdCl 2 + 2Cu Cl 2Cu Cl + 2HCl + 1/2O 2 Cl2 + H O

The catalytic synthesis of acetaldehyde (Wacker process, oxidation of ethylene) C 2 H 4 + PdCl CH 3 CHO + Pd(0) + 2 HCl Nucleophilic attack

The Nobel Prize 2005 (Chauvin, Schrock, Grubbs) Olefin metathesis The Nobel Prize 2005 (Chauvin, Schrock, Grubbs) Grubbs catalyst Schrock catalyst

The metathesis mechanism (Chauvin, 1971)